In 1967, narcolepsy was recognized as a disorder of REM sleep, and its symptoms were attributed to fragments of REM sleep intruding into wakefulness.14 The profound muscle paralysis characterized by REM sleep can help explain cataplexy and sleep paralysis, while dreams experienced during REM sleep help explain the hypnagogic hallucinations.15

The Figure details polysomnography (PSG) recordings of the characteristics of sleep onset in a normal subject vs. a patient with narcolepsy.

Figure: Sleep onset in a normal subject and in a patient with narcolepsy. Note how the normal subject goes from awake to stage 1 (slow eye movements, gradual reduction of chin EMG, theta waves in the EEG) and then to stage 2 (K complexes and sleep spindle). The patient with narcolepsy changes immediately from wakefulness to REM sleep (rapid eye movements, sudden decrease in chin EMG and sawtooth waves in the EEG).16

Narcolepsy with Cataplexy (Narcolepsy Type 1 according to the International Classification of Sleep Disorders) is characterized by a set of four core symptoms, called the “narcoleptic tetrad” (group of four): excessive daytime sleepiness (EDS), cataplexy (a sudden loss of muscle control), hypnagogic hallucinations (vivid hallucinations that occur as a person is falling asleep), and sleep paralysis (while falling asleep or awakening, temporary inability to move, speak or react).17 Only 10-15 percent of these patients have all four symptoms, however.18

Narcolepsy without Cataplexy (Narcolepsy Type 2 according to the International Classification of Sleep Disorders) is diagnosed when the patient presents with hypnagogic hallucinations and/or sleep-onset paralysis, and when polysomnography (PSG) reveals the presence of sleep-onset REM periods (suggestive of narcolepsy) without conclusive evidence of cataplexy.19

This may either be due to the fact that cataplexy has not yet fully developed in these patients or that their narcoleptic symptoms differ etiologically.20 (If cataplexy develops, the disorder should be reclassified as narcolepsy with cataplexy.)

Narcolepsy Due to a Medical Condition is diagnosed when the direct cause of narcolepsy is a comorbid medical disorder. This usually involves mechanisms in the hypothalamus and pons associated with REM sleep, such as hypothalamic pituitary pathology,21 head trauma and brain tumors, hypothalamic sarcoidosis, multiple sclerosis (if the plaques are in the relevant brain regions), and myotropic dystrophy.23